Music amplifiers are at the very heart of each home theater product. As the quality and output power demands of modern speakers increase, so do the requirements of mini amps. It is hard to select an amplifier given the huge number of products and concepts. I will clarify a few of the most common amp designs such as “tube amps”, “linear amplifiers”, “class-AB” and “class-D” in addition to “class-T amps” to help you understand some of the terms commonly used by amplifier suppliers. This guide should also help you figure out which topology is ideal for your precise application.
An audio amp will convert a low-level music signal that often comes from a high-impedance source into a high-level signal that can drive a speaker with a low impedance. The type of element utilized to amplify the signal is dependent on what amp topology is used. Several amps even make use of several types of elements. Typically the following parts are used: tubes, bipolar transistors and FETs. Tube amplifiers used to be popular some decades ago. A tube is able to control the current flow according to a control voltage which is connected to the tube. One dilemma with tubes is that they are not extremely linear while amplifying signals. Aside from the original audio, there are going to be overtones or higher harmonics present in the amplified signal. Therefore tube amplifiers have fairly high distortion. On the other hand, this characteristic of tube amps still makes these popular. A lot of people describe tube amps as having a warm sound versus the cold sound of solid state amps.
Another disadvantage of tube amplifiers, however, is the small power efficiency. The bulk of power which tube amps use up is being dissipated as heat and only a fraction is being transformed into audio power. Tube amps, however, a fairly expensive to produce and therefore tube amps have mostly been replaced with amplifiers making use of transistor elements that are less costly to build.
Solid-state amplifiers employ a semiconductor element, such as a bipolar transistor or FET as opposed to the tube and the first type is known as “class-A” amps. In class-A amps a transistor controls the current flow according to a small-level signal. Some amps utilize a feedback mechanism to minimize the harmonic distortion. Class-A amps have the lowest distortion and generally also the lowest amount of noise of any amplifier architecture. If you require ultra-low distortion then you should take a closer look at class-A models. The main disadvantage is that similar to tube amps class A amplifiers have very low efficiency. Because of this these amps need big heat sinks to dissipate the wasted energy and are typically rather bulky.
Class-AB amps improve on the efficiency of class-A amplifiers. They make use of a number of transistors in order to break up the large-level signals into 2 distinct regions, each of which can be amplified more efficiently. As such, class-AB amps are generally smaller than class-A amps. However, this architecture adds some non-linearity or distortion in the region where the signal switches between those areas. As such class-AB amps usually have higher distortion than class-A amps. Class-D amplifiers improve on the efficiency of class-AB amps even further by utilizing a switching transistor that is constantly being switched on or off. Thus this switching stage barely dissipates any energy and consequently the power efficiency of class-D amps typically surpasses 90%. The switching transistor, that is being controlled by a pulse-width modulator generates a high-frequency switching component that has to be removed from the amplified signal by using a lowpass filter. Due to non-linearities of the pulse-width modulator and the switching transistor itself, class-D amps by nature have amongst the highest audio distortion of any audio amplifier.
In order to solve the dilemma of high audio distortion, modern switching amplifier designs include feedback. The amplified signal is compared with the original low-level signal and errors are corrected. A well-known topology that utilizes this sort of feedback is known as “class-T”. Class-T amplifiers or “t amps” achieve audio distortion which compares with the audio distortion of class-A amps while at the same time offering the power efficiency of class-D amps. Thus t amps can be made extremely small and still achieve high audio fidelity.
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